Electron discharge device



Aug. 11, 1959 K. G. HERNQVlST ELECTRON DISCHARGE DEVICE Filed NOV. 15, 1957 INVENTOR. KARL E. HE-RHUVIST United States Patent ELECTRON DISCHARGE DEVICE Karl G. Hernqvist, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application November 15, 1957, Serial No. 696,842

9 Claims. (Cl. 313-167) This invention relates to electron discharge devices having electrodes which are subject to ion bombardment, and particularly to gas filled electron tubes having grids which are capable of controlling the tube current even to cut-ofif.

One type of recently-developed gas tube generally includes structure similar to the conventional thyratron, but is so dimensioned and designed that complete control of the tube current can be continuously maintained by virtue of the grid voltage. Unlike the conventional thyratron, such tubes can even be cut-01f without reducing the high positive anode voltage by simply applying a sufficient negative voltage to the grid. As herein used, the language negative electrode or negative grid voltage or the like, is meant to describe a condition wherein the voltage on the electrode is negative relative to the potential of the surrounding ionized gas; and, moreover, wherein this relative negative potential is in excess of the threshold potential for sputtering of the particular electrode in question. For conventional electrode !geometry and materials and conventional gases this threshold potential is in the order of 50 to 100 volts. Factors relating to threshold potential for sputtering and to its value for various materials is more fully discussed in the articles Threshold Energies for Sputtering and the Sound Velocity in Metals by G. K. Wehner on pages 633-4 of the February 1, 1954, issue of Physical Review and Controlled Sputtering of Metals by Low-Energy Hg Ions, by G. K. Wehner, on pages 690-704 of the May 1, 1956, issue of Physical Review.

In cutting off a tube such as described above by application of a high negative potential to the grid, serious problems of ion bombardment of the grid are encountered. Positively charged ions are attracted to and bombard the negative grid with suificient force to cause the material of the grid to be sputtered away. The damage ,due to such a sputtering action is first manifested in a poisoning of the cathode and an increase of the electrical leakage between electrodes. Further sputtering may even result in destruction of the sputtered electrode itself.

Such a sputtering action can even occur in a conventional thyratron if operated under certain conditions. In high frequency operation it is sometimes desired to remove the high positive voltage from the anode of a thyratron for a time duration only long enough to permit the grid to regain control of the tube. This may not be a time sufiicient for complete deionization of the tube to take place. Thus, when a negative bias is returned to the grid prior to applying a firing pulse thereto, ions still existing in the tube will be attracted to and bombard the grid. Damage to the cathode and to the grid can result in the same manner as described above.

It is therefore an object of my invention to provide an improved electron discharge device in which a nega tive electrode is protected from harmful sputtering due to ion bombardment thereof.

Briefly, according to my invention the negative elec- N 2,899,586 Patented Aug. 11, 1959 trode of an electron tube is surface wetted with mercury. As a result, ion bombardment will serve to sputter off only the mercury coating, which is readily replaced. The sputtered mercury is not harmful to the cathode.

In the drawings,

Fig. 1 is an elevational view in partial cross-section of an electron tube incorporating my invention; and

Fig. 2 is an elevational view in partial cross-section of another type of electron tube embodying my invention.

Referring to the drawings, Fig. l showsa gas-filled electron tube 10 having an envelope 12. An anode 14, a grid 16, and a cathode 18 are mounted within the envelope on lead-ins 20, 22, and 24, respectively, sealed through the envelope 12. A self-supporting, flat-coiled heater 26 is disposed adjacent the cathode 18. One end of the heater coil 26 is connected to and supported by a lead-in 28 sealed through the envelope 12, and the other end of the heater is connected to and supported by the cathode 18 and cathode lead-in 24. The grid 16 comprises a support ring 30 having a planar mesh portion 32 mounted thereacross. The electron tube 10 is of a type wherein high negative voltages are commonly applied to the grid 16 thereof. As hereinbefore stated, such operation results in ion bombardment of the grid 16 which in turn can result in sputtering of the grid material giving the attendant harmful effects thereof.

According to my invention, the envelope 12 includes a cup-like portion 34 through the bottom of which the grid lead 22 is sealed. The tube 10 is adapted to be operated in an orientation with the cup-like portion 34 at the bottom. A pool of mercury 36 provided in the envelope 12 will thus be contained in the cup-like portion 34 in contact with the grid lead-in 22.

The grid 16 and grid lead-in 22 are made of materials which are easily wet with mercury. For example, nickel, molybdenum, tungsten, tantalum, or platinum are suitable. Thus, with the tube 10 maintained in an operating position, the grid lead-in 22 and the entire grid 16 including the support ring 30 and the mesh 32 will be mercury wetted. Such Wetting results from'the mercury climbing up the lead-in 22 and spreading over the grid 16 due to surface tension. The coating of mercury thus provided over the grid 16 and grid lead-in 22 will serve to protect them from ion bombardment; and ions produced in operation of the tube 10 will not act to sputter away any of the actual material of grid 16 or the grid lead-in 22 when a negative potential is applied thereto. Instead, the mercury coating will itself be sputtered. However, continued surface tension action will serve to replenish the sputtered mercury and continuously maintain a protective mercury coating on both the grid 16 and grid lead-in 22.

In Fig. 2 a gas-filled electron tube 50 somewhat similar to the tube 10 of Fig. 1 is shown to comprise an envelope 52 having a central, re-entrant stem 54 extending from one surface thereof. As such, an annular cup-like recess 56 is provided within the envelope 52. The tube 5!) includes an anode 58, grid 60, cathode 62, and heater coil 64 respectively similar to the anode, grid, cathode, and heater coil of tube 10.

The cathode 62 and heater coil 64 are supported on lead-ins 66 and 68 sealed through the re-entrant stem 54-. The anode 58 is supported in spaced relation to the cathode 62 on a lead-in 70 sealed through the envelope 52 opposite the stem 54. The grid is interposed between the cathode 62 and anode 58 and supported by a pair of rods 72 which extend into the annular recess 56. One of the rods 72 is connected to, or integral with, a lead-in 74 which is sealed through the envelope 52 in the bottom of the annular recess portion 56.

A mercury pool 76 is provided in the annular recess 56 in contact with the grid support rods 72. A porous a 3 body comprising an annular, mercury wettable sponge 78 is disposed on the surface of the mercury pool 76. The sponge 78 may, for example, comprise a body of sintered iron, nickel, or molybdenum powder. Nickel or molybdenum -is preferred because of its superior mercury wettability. As such, the sponge 78 serves to allow the tube 50 to be operated upside down for short intervals of time. terial, the sponge 78 permits the grid support rods 72 and grid 60 to be maintained surface wetted with mercury in the same way as is grid 16 of tube 10. Also, the sponge 78 gives added rigidity to the support of the grid 60. r 7

With reference to Figs. 1 and 2, I have shown and described relatively simple embodiments of my invention. However, it should be appreciated that my inventive concept is not so limited. Although I have shown means whereby'the gridis mercury wet by virtue of a surface tension action, other more complex means in which a more positive application of mercury to the grid is provided will be readily suggested to those skilled in the art. However, I have found the simplified surface tension means to provide an entirely satisfactory wetting of the grid.

It should also be noted that my invention can be incorporated in a tube wherein the gaseous filling is comprised solely of mercury vapor provided by the mercury pool therein, or one in which some other gas serves as the primary ionizable medium. Other suitable gases include any of the so-called noble gases such as argon, neon, and xenon. Of these, xenon is preferred in many applications because of its relatively heavy atomic weight. Such heaviness results in relatively little of the gas being lost by occlusion in the metal parts of the tube.

In some applications it is advantageous according to the invention to add a small amount of hydrogen to the gas filling of the tube. Such an addition will maintain the electrode to be mercury wetted free from any OXi- 2. An electron discharge device comprising a gridelectrode which is' subject to ion bombardment, said electrode having its surface wetted with mercury.

3. An electron tube comprising an envelope containing a grid, a quantity of mercury in said envelope, and means extending into said mercury for wetting the surface of said grid with said mercury.

Yet, being of a mercury wettable ma- 4. A gas filled electron tube comprising an envelope containing a pool of mercury, and cathode, grid, and anode electrodes sealed through and supported in said envelope, said tube being adapted for operation with said mercury pool in contact with a portion of said grid electrode and remote from said cathode and said anode.

5. An electron discharge'device comprising a cathode, a grid, and an anode, said grid having its surface wetted with mercury, said cathode and said anode being free from mercury wetting.

6. An electron discharge device according to claim 5 and including means for maintaining said grid surface in a mercury wetted condition.

7. A gas filled electron discharge device adapted for operation in a given orientation, said device comprising an envelope having a depending cup-like portion, a grid electrode supported within said envelope on a lead-in sealed through said cup-like portion of said envelope, said electrode and said lead-in being made of mercury wettable material, and a quantity of mercury in said cuplike portion, whereby said electrode and said lead-in are maintained in a mercury wet condition due to surface tension of said mercury thereon.

8. A gas filled electron discharge device adapted for operation in a given orientation; said device comprising an envelope; a pool of mercury contained in said envelope; a body of porous material in contact with and substantially covering the surface of said mercury pool; a grid electrode supported within said envelope above said porous body on a lead-in extending through said porous body-into said mercury pool and sealed through said envelope; said electrode, said lead-in, and said porous body comprising mercury wettable material.

9. An electron discharge device comprising a grid, an envelope having a recess, a lead sealed through said envelope in the region of said recess, said lead engaging and supporting said grid in a region remote from said recess, a pool of mercury in said recess only, said grid and lead being made of a material that is wettable by mercury, whereby a portion of said lead is in direct contact with said pool for transferring at least a portion of the mercury thereof to other portions of said lead and'to said grid by the surface tension of said mercury, another electrode in said envelope, and a lead sealed through a portion of said envelope remote from said recess and supporting said another electrode in a region within said envelope remote from said recess, whereby said another electrode is free from mercury wetting by surface tension.

References Cited in the file of this patent UNITED STATES PATENTS 1,929,124 Smith Oct. 3, 1933 2,128,861 'Tonks Aug. 30, 1938 2,165,832 Baruch July 11, 1939 

